The present invention relates to passively shimming magnetic resonance magnets to obtain imaging quality homogeneity in the bore of the magnet.
To create a highly uniform magnetic field with an electromagnet or array of permanent magnets, it is necessary to build the magnet to a carefully specified shape, and to strive to minimize the deviations from the specified shape due to manufacturing variations. The resulting magnets, however, typically require field corrections to achieve the desired level of inhomogeneity, due to deviations of the magnet from the design or due to the presence of ferromagnetic material in the vicinity of the magnet.
To improve field uniformity, correction coils are typically used. These coils are capable of creating different field shapes which can be superimposed on an inhomogeneous main magnetic field to perturb the main magnetic field in a manner which increases the overall field uniformity. Unfortunately, many sets of such coils are typically required. A state of the art magnetic resonance (MR) imaging magnet has between ten and twenty independent sets of correction coils, each with its own power supply to provide the correct current flow. Naturally, these coils add significantly to the cost and complexity of the magnet.
One way of removing the need for correction coils is to shim the magnet passively, using only pieces of iron to bring an initially inhomogeneous field to within imaging homogeneity specifications. With the iron placed inside the bore of the magnet a minimal addition to the size and weight would be required. A passively shimmed magnet would be cheaper and more reliable than the typical set of correction coils presently used.
The primary difficulty in implementing such a shimming approach lies in predicting the locations and sizes of iron pieces required to shim the field. Electromagnetic coils are generally designed to produce certain terms of a spherical harmonic expansion. Such a design criteria is difficult to implement with passive shims because the permeability of iron cannot be reversed, whereas a current reversal through a coil can be used to obtain a field reversal in a correction coil. Additionally, the size and complexity of the groups of shim pieces which would be required to produce a single harmonic would not make this approach feasible. Since magnetic coupling between the shims is also a complicating factor, shimming with large pieces which inevitably become physically close to one another increases the difficulty in properly shimming the magnet.
Passive shimming is currently used to correct large deviations in magnetic fields that cannot be corrected by the available correction coils alone. The passive shimming is accomplished by placing a piece of iron in an appropriate place outside the magnet. The desired level of field uniformity can then be achieved by the correction coils.
It is an object of the present invention to provide a method of passively shimming a magnetic resonance magnet to achieve a level of field inhomogeneity required for magnetic resonance imaging, without the use of correction coils.
It is a further object of the present invention to provide a method of determining the optimum axial and circumferential locations of shims to bring the field homogeneity to a level consistent with magnetic resonance imaging, using only pieces of ferromagnetic material.
It is a still further object of the present invention to provide a method of minimizing the total field inhomogeneity and not selected harmonics.